The forward assist on an M16 or AR-15 type rifle is a button, usually located near the bolt closure that, when actuated, will push an associated bolt carrier forward, ensuring that the bolt is locked and to assist in forcing a round into the battery in the event of the bolt not fully closing. In order to ensure that an associated extractor component is clipped around the rim of the shell, the forward assist is usually struck rather than pushed.
The forward assist is commonly incorporated into standard loading procedure to prepare a weapon for a life threatening situation, or to close the bolt when the weapon is excessively dirty. The forward assist can also be used to close a bolt that was gently let down, rather than being released under full spring compression, and in order to keep the noise of closing the bolt to a minimum.
In acknowledgement of the frequency with which a weapon jams when taken outside of the clean environment of the test range, the forward assist was implemented to save the operator the potential danger of aiming the rifle and pulling the trigger and the rifle not discharging the shell due to the bolt not being fully forward (a safety arrangement called a “safety sear” stopping the hammer from being released and the weapon firing, because of the dangers of firing with the bolt not fully closed).